In hardware terms, a telephone link comprises various items of equipment such as network terminals, line terminals, subscriber lines, local switching centers and the public switched telephone network. More precisely, each subscriber set, for example a telephone set or a modem linked to a computer, is connected, by way of a subscriber tap, to a network terminal, essentially consisting in hardware terms of a specific card, such as for example a so-called TNR card marketed by various companies such as ALCATEL, SAT, SIEMENS or PHILIPS. Each network terminal is linked by a subscriber line to a line terminal located at a local switching center. In hardware terms, the line terminal also includes a specific card, in particular a TABN card marketed by the same companies, and including eight inputs/outputs so as to manage eight subscriber lines. The two local switching centers associated with the telephone link connecting the two relevant subscriber sets are mutually linked by a public switched telephone network.
The transfer and exchange of data and documents carried out by means of these subscriber sets have over the course of this last decade become methods of everyday communication between geographically remote individuals and/or entities. This worldwide electronic communication process has accelerated even more over the course of recent years with the planet-wide development of the Internet network. Through these technologies, without the apparent intervention of an intermediary and almost in real time, the economic world exchanges and transmits information which may exhibit higher or lower degrees of confidentiality.
The commonplace use of these novel forms of communication has highlighted the problem of making secure the exchanges between opposite parties, that is to say between a sender and his intended destination. One of their chief objectives is then to preclude the possibility of the information which they transfer by way of the public switched telephone network being picked up and used without their knowledge by third parties. A solution to this problem could consist in installing encryption/decryption means in the local network of each subscriber, that is to say upstream of the subscriber tap. In this case, the information exchanged between the two subscriber sets is end-to-end encrypted between each encryption/decryption means installed within the subscriber's, local network. Nevertheless, such a solution has numerous drawbacks.
It requires firstly that the encryption/decryption hardware and the corresponding software implemented within this hardware be perfectly mutually compatible. In practice, the hardware and the software will have to be virtually identical. Now, this is difficult to achieve, given the very great disparity which may exist between the various subscribers. Moreover, such a solution requires that a third party organization manage the allocation of the various encryption keys to the subscribers. Furthermore, the communication of the encryption keys between this third party organization and each of the subscribers has likewise to be secure, this constituting a further difficulty.
Finally, in the event that a secured subscriber wishes to contact a non-secured subscriber, the former has to provide means internal to his local network and capable of disconnecting its own encryption/decryption means.
The invention aims to afford a solution to these problems.